Story related, cannot be missed.
This achievement is unlocked after defeating the second boss at the end of the second act of the game. The second boss is a giant hamster that initially rolls up to you in a huge wheel with spikes on it. Luckily you can drop through the floor in certain parts of the area and shoot upwards at him, but he will not be able to attack you if you stay below ground level, making this battle relatively easy. You will be attacked by smaller enemies who can follow you below ground level, but they are all lesser enemies and don't pose too much of a threat.